Structural basis of Schnyder corneal dystrophy Schnyder corneal dystrophy (SCD) is a rare but severe visual disease because most SCD patients over 50 years of age require corneal transplants. SCD is characterized by the progressive cholesterol deposition and opacification in the cornea that lead to visual loss. SCD is caused by mutations in the UBIAD1 protein, a prenyltransferase that binds to HMGCR, which is the rate-limiting enzyme in cholesterol synthesis. SCD mutants cannot release HMGCR for degradation, and the constitutive activity of HMGCR results in cholesterol accumulation. Understanding the molecular basis of this binding and release cycle that triggers HMGCR degradation is crucial to control cholesterol deposition in the cornea of SCD patients. However, it remains unclear how UBIAD1 interacts with HMGCR, how their structural changes regulate the HMGCR release, and how SCD mutants are blocked for this release. The technical challenge is that both UBIAD1 and HMGCR are human membrane proteins and their hydrophobic and dynamic nature presents challenges for traditional methods. The objective of this application is to elucidate underlying structural mechanisms of SCD mutations and HMGCR regulation by an innovative and integrative approach that merges membrane biology with mass spectrometry (MS), electron microscopy, and molecular dynamics simulation. Our hypotheses are 1) the catalysis of UBIAD1 is associated with conformational changes at its HMGCR-binding interface that trigger the release of HMGCR, and 2) SCD mutants are conformationally trapped against this release. The basis for these hypotheses is built on extensive preliminary data. We have made early discovery that UBIAD1 is the causative gene of SCD, isolated several SCD mutations in UBIAD1, and identified HMGCR as a binding partner to UBIAD1. More recently, we determined the crystal structures of a UBIAD1 homolog, which is the first in this superfamily of prenyltransferases; the structures suggest that UBIAD1 catalysis is associated with conformational changes at a putative HMGCR-binding interface, thus linking UBIAD1 catalysis with HMGCR release. Testing our hypotheses will be guided by two specific aims: (1) to probe the regulated binding of UBIAD1-HMGCR and the SCD-interfered release in living cells with MS-based footprinting; and (2) to understand the regulation of UBIAD1- HMGCR structure and the mechanism of SCD mutants in purified systems. Elucidating these structural mechanisms is promising to the rational design of drugs that specifically disrupt the UBIAD1-HMGCR interface to release HMGCR for degradation. Our long-term goal is to develop topical medication to control cholesterol deposition in SCD patients, eventually aiming to prevent visual loss and the need for corneal transplant surgery. Although SCD is a rare disease, we envision that understanding of the UBIAD1-HMGCR pathway may provide new insights into the underexplored lipid metabolism processes and related diseases in the eye. Because the histopathologic changes of SCD are similar to atherosclerosis, our work also has implications for the treatment of systemic hypercholesterolemia, therefore affording broad-reaching prospects beyond the eye.